The use of polymeric materials for medical devices is an area where vast improvements in polymeric materials have evolved and are still evolving. Physical properties of these polymers can be fine tuned for use in different environments or to behave in a predictable manner. Polymers for use in fabricating IOLs need to be adapted allowing for smaller incisions during implantation.
In addition to polymers with physical properties adapted for use in the optic region, polymeric material can be adapted for various applications elsewhere. Various medical devices other than optic implants can incorporate elastomeric polymeric material, for example, implantable medical device coatings, breast implants, prosthetic joins and other body augmentation implants. These types of applications each require polymeric material that may be vastly different from another. The ability of a skilled artisan is necessary to fine tune the physical properties of the polymers.
In addition to medical devices and medical device coatings, polymeric material can be incorporated into topical formulations. A low degree of polymerization can be used, for example, to form a more liquid polymeric material which can be useful in such formulations as eye drops or hair sprays. Increasing the degree of polymerization can cause the polymer to become more viscous wherein the polymer may be useful in skin creams or lotions. The degree of polymerization can be tailored for the appropriate application and various other variations are possible.
In the area of ophthalmic devices, IOLs have been designed for ever smaller incisions in the eye. Elastomeric IOLs are typically implanted using inserters to roll or fold the IOL, insert the IOL into the capsular sac, and then allow the IOL to unfold once inside. Occasionally, the fold of the IOL before insertion may result in permanent deformation, which adversely affected the implant's optical qualities. Further, while foldable IOLs have eliminated the need for the large incision, foldable IOLs are not without drawbacks. In particular, both non-deformable and foldable IOLs are susceptible to mechanical dislocation resulting in damage to the corneal endothelium.
Another approach to small incision IOL implantation uses an elastomeric polymer that becomes pliable when heated to body temperature or slightly above. Specifically, the IOL is made pliable and is deformed along at least one axis, reducing its size for subsequent insertion through a small incision. The IOL is then cooled to retain the modified shape. The cooled IOL is inserted into the capsular sac and the natural body temperature warms the IOL at which point it returns to its original shape. The primary drawback to this type of thermoplastic IOL is the limited number of polymers that meet the exacting needs of this approach. Most polymers are composed of polymethylacrylate which have solid-elastomeric transition temperatures above 100° C. Modifications of the polymer substrate require the use of plasticizers that may eventually leach into the eye causing harmful effects.
Dehydrated hydrogels have also been used with small incision techniques. Hydrogel IOLs are dehydrated before insertion and naturally rehydrated once inside the capsular sac. However, once fully rehydrated the polymer structure is notoriously weak due to the large amount of water absorbed. The typical dehydrated hydrogel's diameter will expand from 3 mm to 6 mm resulting in an IOL that is 85% water. At this water concentration the refractive index (RI) drops to about 1.36, which is unacceptable for an IOL since lower RI materials require the optic to be thicker to achieve a given optical power.
Modern acrylate IOLs generally possess excellent mechanical properties such as foldability, tear resistance and physical strength. Acrylate IOLs also are known to possess desirable optical properties (transparency, high refractive index, etc.) and biocompatibility. While pure acrylic IOLs have desirable mechanical, optical and biological properties, they may have unacceptable molecular response times such that the folded or compacted IOL may not unfold as quickly as desired. A pure acrylate IOL fabricated to have a relatively fast molecular response time may be extremely tacky and lack the desired mechanical strength. In this case, the resulting IOL may tear and/or the resulting self-tack can unfolding difficult.
Pure silicone IOLs generally possess excellent mechanical, optical and biological properties similar to pure acrylate IOLs. Unlike acrylic IOLs, silicone IOLs generally possess faster molecular response times. In fact, the silicone IOLs may be so responsive that when folded small enough to be inserted through a 3 mm or smaller incision, the stored energy can cause the IOL to unfold more quickly than desired.
There is a need for a polymeric material with a molecular response time which makes it suitable for use near fragile body tissues, such as within the eye of a subject. There is also a need for ophthalmic devices in which one polymeric material is useful for both low modulus and high modulus elements of a single device to, inter alia, simplify the multi-part polymeric article manufacturing process and create better integrated multi-part polymeric articles in which the various elements of the device have a common value of a property such as a refractive index, but a different value of another property such as modulus, tensile strength, resiliency, or the like.